Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g., AlGaAs) have been developed since the mid-80's (K. Kinoshita et al., IEEE J. Quant. Electron. QE-23, 882 (1987)). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (K. D. Choquette et al., Proc. IEEE 85, 1730 (1997)). With the success of these near-infrared lasers in recent years, attention has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (C. Wilmsen et al., Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many fruitful applications for visible lasers, such as display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (T. Ishigure et al., Electron. Lett., 31, 467 (1995)). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to create viable laser diodes (either edge emitters or VCSELs), which span the visible spectrum.
In the effort to produce visible wavelength VCSELs, it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems. Organic materials have properties making them suitable for gain media in these lasers, such as low scattering/absorption losses and high quantum efficiencies. Organic lasers offer the advantage over inorganic systems in that they are relatively inexpensive to manufacture and can be made to emit over the entire visible range.
The usual route for making a manufacturable laser diode system is to use electrical injection rather than optical pumping to create the necessary population inversion in the active region of the device. This is the case for inorganic systems, since their optically pumped thresholds for broad-area devices are on the order of 104 W/cm2 (P. L. Gourley et al., Appl. Phys. Lett. 54, 1209 (1989)). Such high power densities can only be obtained by using other lasers as the pump sources, precluding that route for inorganic laser cavities. Unpumped organic laser systems have greatly reduced combined scattering/absorption loss (˜0.5 cm−1) at the lasing wavelength, especially if a host-dopant combination is used as the active media. As a result, optically pumped power thresholds below 1 W/cm2 should be attainable, especially when a VCSEL-based microcavity design is employed in order to minimize the active volume (which results in lower thresholds). At these threshold power levels it becomes possible to optically pump organic-based vertical laser cavities using incoherent light-emitting diodes (LEDs). This result is highly significant for amorphous organic laser systems, since driving them by electrical injection has, to this date, been unobtainable mainly as a result of the low carrier mobility of organic materials (N. Tessler et al., Appl. Phys. Lett., 74, 2764 (1999)).
It is possible to provide an organic surface-emitting laser arrangement that is particularly suitable to permitting optimization of the organic active region, improving power conversion efficiency, and removing unwanted spontaneous emission by constructing an organic vertical cavity surface-emitting laser device (Organic VCSEL) that produces light, for example, see U.S. Pat. Nos. 6,728,278, 6,690,697, US 2004/0076203 and references cited therein.
Desirably the device comprises: a bottom dielectric stack reflective to light over a predetermined range of wavelengths; an organic active region for producing laser light, and including emissive material; a top dielectric stack spaced from the bottom dielectric stack and reflective to light over a predetermined range of wavelengths; a pump-beam light is transmitted and introduced into the organic active region through at least one of the dielectric stacks; and the organic active region includes one or more periodic gain region(s) and organic spacer layers disposed on either side of the periodic gain region(s) and arranged so that the periodic gain region(s) is aligned with the antinodes of the device's standing wave electromagnetic field, and wherein the spacer layers are substantially transparent to the laser light.
To enable a large area laser structure, which emits single- or multi-mode (a few modes), it is advantageous to construct a two-dimensional laser array device as shown schematically in FIG. 1. FIG. 2 shows a top view of the two-dimensional laser array device where on the surface of the VCSEL there are lasing pixels 200 separated by interpixel regions 210. In general the lasing pixels 200 are regions where the net gain of the device is larger than in the interpixel regions 210. As applied to two-dimensional inorganic laser arrays, a route for varying the net gain of the device is to modulate the reflectance of the top dielectric stack by either adding metal (E. Kapon and M. Orenstein, U.S. Pat. No. 5,086,430) or by deep etching into the top dielectric stack (P. L. Gourley et al., Appl. Phys. Lett., 58, 890 (1991)). In both inorganic laser array cases, the laser pixels were on the order of 3-5 μm wide (so as to enable single-mode action) and the interpixel spacing was 1-2 μm. This technique can be applied to organic laser systems. The metal can be deposited by either conventional thermal evaporation, sputtering or electron-beam deposition techniques. The metal is patterned using standard photolithographic and etching techniques, thus forming a two-dimensional array of circular metallic pits on the surface of the substrate. The metal is removed from the laser pixel regions, while the metal remains in the interpixel regions. As a result, the reflectance of the device is smaller where the metal remains. However, it is very difficult to perform micron-scale patterning in a reproducible fashion on the laser structure once the organic layers have been deposited.
In another approach, the lower net gain regions are created by locally spoiling the emissive properties of the periodic gain region(s). This process uses standard photolithographic masks and UV exposing apparatus in order to create a patterned UV exposure of the periodic gain region(s). Since organic media is sensitive to high intensity UV radiation (for example the mercury arc lamp's i-line at 365 nm), this technique works very effectively to lower the emissive intensity of the UV exposed areas of the periodic gain region(s) by irreversibly changing the properties of the emissive material. In this case the UV exposed areas correspond to the interpixel region, while the unexposed areas correspond to the laser pixels. However this method has not been fully established and alternative methods of forming the laser pixel pattern are highly desirable.
Many methods have been described for formation of a micro-pattern or image, including those using the lithographic process, for example see M. Bowden, J. Electro. Chem. Soc., 128, 195C-214C (1981) for a review. Other methods include those reported in U.S. Pat. Nos. 5,270,727, 4,782,006, 4,251,622, EP 0511403, JP 2002/293039, JP 2000/206645, JP 08035184, JP 06347633. However, many of these methods are not readily applied to the type of patterning needed in an Organic VCSEL because they leave the surface of the pattern pitted and not smooth, which can cause undesirable optical problems including light scattering. In some cases several components are required to generate the pattern and it would be desirable to find a simpler method.
B. K. Barnett and T. D. Roberts, J. Chem. Soc. Chem. Comm., 758 (1972) have described the quantitative photodecarbonylation of formanilides to give anilines. This technique has not been suggested for amino substituted dye molecules or for imaging applications.
Thus, there remains a need to find new methods to form filters, images or information records having a smooth surface and in particular to form a pixel region pattern for use with an Organic VCSEL that would offer ease of manufacturing.